1. Field of the Invention
The present invention relates to interconnection networks, and, more particularly, to multichannel switches using spatial light modulators for reconfigurable switching of multiple inputs to multiple outputs.
2. Description of the Related Art
Reconfigurable interconnection of several high data rate transmitters to several receivers is a cumbersome and technically difficult task. However, reconfigurable interconnection networks (including reconfigurable crossbar switches) underlie a variety of devices such as high computation rate parallel computing architectures where numerous processors route information to each other or share common resources, communications switching as in telephone switching centers, and aircraft fiber optic busses that require configurability to allow redundancy for fault tolerance and the ability to share several sensors with several processors. Crossbar switches are, in effect, N.times.M arrays of switches for connecting each of N inputs to any or all of M outputs. Some applications, such as aircraft avionics systems, supercomputer processor interconnects, and local area networks require only moderate size arrays (N,M.apprxeq.20) but demand the high bandwidth and noise immunity found in optical interconnections plus mechanical ruggedness. Easy reconfigurability of the crossbar switch is desirable in other applications such as mobile and fixed networks, industrial controls, and interconnections between local area networks and mainframe computers.
Very large scale integration in semiconductor devices is also leading towards the greater use of parallelism. Parallelism requires some sort of interconnection between the processing elements and this introduces a trade off between speed and the ability to handle a wide range of algorithms. For example, a complex interconnection network provides some flexibility at the expense of speed, and high speed may be achieved by means of fixed interconnections for a specific algorithm. The problem is to achieve very high speed by efficiently using a large number of processing elements and at the same time retain extremely high algorithm flexibility. Efficiency for parallel processing is `the gain in speed versus that using a single processor of the same type` divided by `the number of processors`. Also, the complexity of the processing elements relates to the degree of parallelism obtainable; sophisticated computations tend to have parts that are not parallelizable at a coarse level. The overall speed is dominated by the parts which are non-parallelizable at a coarse level. And a large number of fast elementary processors places a considerable communication burden on the interconnection between processors. There is a need for parallel processor interconnections that possess simple reconfigurability.
Fixed interconnections limit the range of algorithms which may be efficiently implemented. Systolic configurations, such as those in development at Carnegie-Mellon University (Kung H. T., Why Systolic Architectures?, IEEE Computer, Jan. 1982 p. 37-46), use algorithm structure to reduce memory and instruction fetches. This reduces communication time and permits large numbers of processors to be efficiently used in parallel. However, the algorithm constraints are significant because of the fixed interconnections.
Algorithm flexibility may be achieved by compelx reconfigurable interconnection networks, and a prototype system having 8 processors and using a Banyan switch is in operation at the University of Texas at Austin (Browne J. C., Parallel Architectures for Computer Systems, Physics Today, Vol. 37, No. 5, May 1984). A Banyan is a multichannel switch composed of levels of 2.times.2 switches. However, this type of reconfigurability introduces large delays and high control overhead in most proposed systems and this restricts the number of processors and the speed of the system.
Small commercial crossbar switches made entirely of semiconductor devices have recently become available, the AS8840 chip from Texas Instruments. The AS8840 is a 16-port crossbar integrated circuit which is dynamically reconfigurable; each of the ports handles a nibble (four bits) bidirectionally.
However, electromagnetic interference has been difficult to control in most electronic implementation of reconfigurable crossbars switches, and most electronic implementations also require a dense interconnection scheme that is either difficult or time consuming to fabricate.
Optical crossbar switches have been suggested to overcome the electromagnetic interference and bandwidth problems. Indeed, the communications industry makes widespread use of optical fibers and is developing optical switching devices to avoid conversion to electronics and back for switching purposes. Optics has been suggested for communication with VLSI to overcome the bandwidth pin limitations and edge connection constraints; see Goodman J. W., Leonberger F. J., Kung S. Y. and Athale R. A, Optical Interconnections for VLSI Systems, Proc. IEEE, Vol. 72, No. 7, July 1984, p. 850-866.
D. Grant et al., An Optical Phased Array Beam Steering Technique, 1971 Proceedings of the Electro Optic System Design Conference pages 259-264, describes reflection of colimated light for a membrane spatial light modulator which is passed through a sampling mask and the phase variations caused by the pixels of the spatial light modulator combine to form a single spot on a receiver array. Varying the pixel deformations controls the phase variations and thereby steers the spot across the receiver array.
A. McAulay et al. have discussed design considerations and light budget for a large (N&gt;100) optical crossbar switch using DMD arrays as the reconfigurable switch element; this appears in the milestone and final reports for DARPA Contract N00014-85-C-0755 administered by the Office of Naval Research and in R. Cohn, Link Analysis of a Deformable Mirror Device Based Optical Crossbar Switch, Proc. SPIE 825 (1987).
However, the known optical interconnection networks for high speed data transmission are limited primarily by the performance of the optical switching elements which tend to restrict wavelength, polarization, switching speed, dynamic range, and switch topology. Additionally, current systems suffer from significant limitations in the areas of mechanical ruggedness, and optical power handling efficiency.